do_wait() optimization: do not place sub-threads on task_struct->children list

Thanks to Roland who pointed out de_thread() issues.

Currently we add sub-threads to ->real_parent->children list.  This buys
nothing but slows down do_wait().

With this patch ->children contains only main threads (group leaders).
The only complication is that forget_original_parent() should iterate over
sub-threads by hand, and de_thread() needs another list_replace() when it
changes ->group_leader.

Henceforth do_wait_thread() can never see task_detached() && !EXIT_DEAD
tasks, we can remove this check (and we can unify do_wait_thread() and
ptrace_do_wait()).

This change can confuse the optimistic search in mm_update_next_owner(),
but this is fixable and minor.

Perhaps badness() and oom_kill_process() should be updated, but they
should be fixed in any case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Ratan Nalumasu <rnalumasu@gmail.com>
Cc: Vitaly Mayatskikh <vmayatsk@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 202a0ba..5b2959b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1291,7 +1291,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	}
 
 	if (likely(p->pid)) {
-		list_add_tail(&p->sibling, &p->real_parent->children);
 		tracehook_finish_clone(p, clone_flags, trace);
 
 		if (thread_group_leader(p)) {
@@ -1303,6 +1302,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 			p->signal->tty = tty_kref_get(current->signal->tty);
 			attach_pid(p, PIDTYPE_PGID, task_pgrp(current));
 			attach_pid(p, PIDTYPE_SID, task_session(current));
+			list_add_tail(&p->sibling, &p->real_parent->children);
 			list_add_tail_rcu(&p->tasks, &init_task.tasks);
 			__get_cpu_var(process_counts)++;
 		}